The Development of a Photoelastic Floating Element Force Balance

We present a new floating element drag balance design that uses an optical measurement of the force using photoelastic stress analysis. The force sensing element consists of pins embedded in photoelastic polyurethane pads, which generate internal stress when the floating element is loaded. A series of known loads and their corresponding fringe patterns allow a calibration matrix to be derived using a 5th-order polynomial model solved by a least square regression. FEA is carried out to validate the proposed photoelastic method. The balance then measured the lift curve of the NACA0015 wing at 5m/s and 7m/s. A comparison of the photoelastic balance and an ATI Mini 40 load cell showed differences of 6%. This optical approach enables accurate measurements with inexpensive and simple components inside the sensor. The balance design minimizes the element's misalignments relative to the surface and can be sealed to avoid horizontal buoyancy forces from adverse pressure gradients, crucial for wall shear stress measurements. It can also be tailored for different load cases and scaled to fit complex setups across various magnitudes. A photoelastic balance is a simple, effective, and versatile means of force measurement, able to measure forces in a time-averaged manner and instantaneously.